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| United States Patent |
5,080,754
|
|
Francis
, et al.
|
January 14, 1992
|
Method for reducing brightness reversion in lignin-containing pulps and
article of manufacture thereof
Abstract
A method and article of manufacture thereof is provided for reducing
brightness reversion in bleached lignin-containing pulps or newsprint by
the treatment of the bleached lignin-containing pulp or newsprint with a
compound which donates a hydrogen atom to a photo-excited group or free
radical more easily than does the lignin contained in the treated pulp.
The hydrogen donating compounds include compounds with certain formyl
functionality, including formate salts such as sodium, magnesium and
calcium formate, formamides, formic acid esters, and formylurea. In one
embodiment, calcium carbonate is added to enhance the activity of the
formyl compound. The further addition of a trace amount of superoxide
anion quencher such as copper sulfate or ascorbic acid is also provided to
still further enhance the activity of the formyl compound, especially
formate salts.
| Inventors:
|
Francis; Raymond C. (DeWitt, NY);
Dence; Carlton W. (Syracuse, NY);
Alexander; Thomas C. (Clay, NY)
|
| Assignee:
|
The Research Foundation of State University of NY (Albany, NY)
|
| Appl. No.:
|
557153 |
| Filed:
|
July 20, 1990 |
| Current U.S. Class: |
162/9; 162/72; 162/76; 162/150; 162/157.6; 162/158; 162/160; 162/181.2 |
| Intern'l Class: |
D21C 009/00 |
| Field of Search: |
162/150,158,160,166,76,72,9,157.6,135,181.2
|
References Cited
U.S. Patent Documents
| 2005378 | Jun., 1935 | Kiel | 162/160.
|
| 2285490 | Jun., 1942 | Broderick | 162/158.
|
| 2622960 | Dec., 1952 | Woods | 162/158.
|
| 3069311 | Dec., 1962 | Harpham et al. | 162/157.
|
| 3723425 | Mar., 1973 | Ackermann et al. | 260/240.
|
| 3870544 | Mar., 1975 | Shisko | 117/60.
|
| 3959570 | May., 1976 | Jacquelin et al. | 162/126.
|
| 4124439 | Nov., 1978 | Dessauer | 162/166.
|
| 4204055 | May., 1980 | Lesas et al. | 162/157.
|
| 4871423 | Oct., 1989 | Grimsley et al. | 162/160.
|
| 4904342 | Feb., 1990 | Arnoldy et al. | 162/76.
|
| Foreign Patent Documents |
| 0280332 | Aug., 1988 | EP.
| |
| 2026963 | Oct., 1970 | DE | 162/158.
|
Primary Examiner: Chin; Peter
Attorney, Agent or Firm: Heslin & Rothenberg
Claims
What is claimed is:
1. A method for reducing brightness reversion of bleached pulps containing
lignin comprising the steps of forming said pulps into a sheet, and
treating said sheet with a compound other than forminc acid, said compound
having formyl functionality of the type shown in FIG. I,
##STR3##
wherein O is oxygen, C is carbon, H is hydrogen and R is any functional
group having an element bonded to C, said element being selected from the
group consisting of nitrogen and oxygen.
2. A method for reducing brightness reversion of pulps containing lignin
for use as newsprint comprising the steps of forming said pulps into a
sheet of newsprint paper, and treating said newsprint sheet with a
compound other than formic acid, said compound having formyl functionality
of the type shown in FIG.
##STR4##
wherein O is oxygen, C is carbon, H is hydrogen and R is any functional
group having an element bonded to C, said element being selected from the
group consisting of nitrogen and oxygen.
3. A method for reducing brightness reversion as recited in claims 1 or 2
wherein said sheet is further treated with calcium carbonate.
4. A method for reducing brightness reversion as recited in claims 1 or 2
wherein said sheet is further treated with a trace amount of superoxide
anion quencher.
5. A method for reducing brightnes reversion as recited in claim 4 wherein
said superoxide anion quencher is copper sulfate or ascorbic acid.
6. A method for reducing brightness reversion of bleached pulps containing
lignin, comprising the steps of forming said pulps into a sheet, and
treating said sheet with a compound selected from the group consisting of
formate salts, formamides, formic acid esters and N formylurea.
7. A method for reducing brightness reversion of pulps containing lignin
for use as newsprint comprising the steps of forming said pulp into a
sheet of newsprint paper, and treating said sheet with a compound selected
from the group consisting of formate salts, formamides, formic acid
esters, and N-formylurea.
8. The method for reducing brightness reversion as recited in claims 6 or 7
wherein the formate salt is sodium formate, calcium formate, or magnesium
formate.
9. The method for reducing brightness reversion as recited in claims 6 or 7
wherein the formamide is N,N-dibutylformamide.
10. The method for reducing brightness reversion as recited in claim 8
wherein said sheet is further treated with calcium carbonate.
11. The method for reducing brightness reversion as recited in claim 8
wherein said sheet is further treated with a trace amount of superoxide
anion quencher.
12. The method of reducing brightness reversion as recited in claim 11
wherein the superoxide anion quencher is copper sulfate or ascorbic acid.
13. A sheet of paper comprising bleached pulps containing lignin, which
pulps have been treated with a compound other than formic acid, said
compound having formyl functionality of the type shown in FIG. I,
##STR5##
wherein O is oxygen, C is carbon, H is hydrogen and R is any functional
group having an element bonded to C, said element being selected from the
group consisting of nitrogen and oxygen.
14. A sheet of paper comprising pulps containing lignin for use as
newsprint, which pulps have been treated with a compound other then formic
acid, said compound having formyl functionality of the type shown in FIG.
I,
##STR6##
wherein O is oxygen, C is carbon, H is hydrogen and R is any functional
group having an element bonded to C, said element being selected from the
group consisting of nitrogen and oxygen.
15. The sheet of paper as recited in claim 13 or 14 wherein said compound
is selected from the group consisting of formate salts, formamides, formic
acid esters and N-formylurea.
16. The sheet of paper as recited in claim 15 wherein the formate salt is
sodium formate, calcium formate or magnesium formate.
17. The sheet of paper as recited in claim 15 wherein the formamide is
N,N-dibutylformamide.
18. The sheet of paper as recited in claim 15 wherein the formic acid ester
is hexyl formate.
19. The sheet of paper as recited in claim 16 wherein the sheet is further
treated with calcium carbonate.
20. The sheet of paper as recited in claim 16 wherein the sheet is further
treated with a superoxide anion quencher.
21. The sheet of paper as recited in claim 20 wherein the superoxide anion
quencher is copper sulfate or ascorbic acid.
Description
BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention relates to the field of paper manufacturing and, more
particularly, to maintaining the brightness of bleached pulps or paper
containing lignin. Maintaining brightness is also known in the industry as
retarding or reducing brightness reversion.
II. Description of the Prior Art
Paper or pulps which contain lignin or ligno-cellulose have historically
been bleached and continue to be bleached to obtain whiteness. The major
drawback of bleached lignin-containing pulps is that they are easily and
extensively darkened by light irradiation. This limits their use in
various grades of printing papers.
Paper or pulp used in making newsprint is generally not bleached. If it is
bleached, it is relatively mildly bleached as compared to higher quality
paper. Therefore, such unbleached or mildly bleached pulps have a darker
quality than bleached pulps. Newsprint has a relatively high lignin
content and, therefore, has a tendency to become even darker when exposed
to light. The invention can be used to prevent darkening of all types of
lignin-containing pulps including bleached and unbleached pulps, such as
newsprint.
The whiteness of paper is estimated by brightness measurements which are
based on the reflectance of light having an average wavelength of 457 nm.
An Elrepho brightness meter is one type of the instrument used to measure
paper brightness. A low brightness (40% Elrepho) indicates brown or dark
paper, while 90% Elrepho typifies white paper. Lignin-rich pulps have
brightness values in the range of 50-70% Elrepho, depending on the wood
species used and the pulping process. These pulps can be bleached to
70-90% brightness using known brighteners such as hydrogen peroxide,
sodium borohydride or sodium dithionite. Hydrogen peroxide is normally
used when a brightness of more than 70% is required. A problem associated
with bleached lignin-rich pulps is that they may darken by as much as 20
Elrepho points when exposed to natural sunlight during exposure over a
period of only one day.
Several methods are presently known to decrease brightness reversion in
pulps containing lignin. The disadvantage of these methods is that they
add significantly to the cost of the paper manufacturing process and are
less effective than the invention.
One known method described in European Patent No. 0 280 332 (Agnemo et al.)
consists of serial treatments to reduce the carbonyl groups
(photosensitizers) contained in the pulp to alcohol groups. In addition,
the process includes alkylation of the phenolic hydroxyl groups in the
lignin, from which hydrogen atoms are abstracted, by the use of an
alkaline propylene oxide. Addition of fluorescent compounds that absorb or
reflect the ultraviolet light which would otherwise excite photosensitizer
groups is also disclosed.
There has not been any method taught in the prior art in which additives
were specifically selected for their ability to donate hydrogen atoms to
reactive chemicals present in the bleached pulp upon exposure to light.
Formic acid, which is a formyl compound, has been used as a catalyst in the
brightening process of organic materials (i.e., U.S. Pat. No. 3,723,425,
Ackermann et al.). However, formic acid should not be used to retard
brightness reversion because of its acidity which is known to accelerate
brightness reversion due to light irradiation (Lee et al., J. Amer. Inst.
Conserv. 28:1 (1989)).
Of the known processes for maintaining brightness or reducing brightness
reversion, none have the capacity to reduce brightness reversion as
effectively as the invention, while at the same time minimizing the
additional cost of production of the paper. Of the known processes, none
is available at a low cost and none has outstanding whiteness
stabilization qualities. A need thus persists for an efficient and low
cost paper manufacturing process which reduces the brightness reversion of
bleached pulps containing lignin.
SUMMARY OF THE INVENTION
This need is satisfied and the shortcomings of the prior art are overcome,
in accordance with the principles of the present invention.
The present invention discloses that certain formyl compounds, which are
good hydrogen donors, can be added to the papermaking process to retard
brightness reversion.
The claimed method for reducing brightness reversion in bleached pulps
containing lignin comprises the steps of forming said pulps into a paper
sheet and treating said paper sheet with a compound having formyl
functionality. It is believed that formyl compounds donate a hydrogen atom
to a photo-excited group or free radical more easily than lignin donates a
hydrogen atom to the same photo-excited group or free radical. The
photo-excited group is created by light irradiation of the paper and may
react in such a way as to generate free radicals.
The compounds which can be used as good hydrogen donors are those compounds
with a hydrogen bond which is weaker than the hydrogen bond of the
ligno-cellulose complex but which are not too strongly acidic. Good
hydrogen donating compounds, such as those with a certain type of formyl
functionality, include formate salts, formamides, formic acid esters and
n-formylurea. These compounds are best described by FIG. I below,
##STR1##
wherein O is oxygen, C is carbon, H is hydrogen and R is any functional
group having an element bonded to C, said element being selected from the
group consisting of nitrogen and oxygen. If a strong acidic formyl
compound is used, it should first be converted to a metal salt or formyl
ester before application to the pulp.
The method of use of the formyl compound is enhanced by adding calcium
carbonate to the surface of the paper. In particular, the addition of
calcium carbonate to assist formate salts has shown good results in
reducing brightness reversion. Trace amounts of a superoxide anion
quencher such as copper sulfate or ascorbic acid are also used to assist
the formate salts (i.e., sodium formate, calcium formate, magnesium
formate) in stabilizing pulp brightness.
Accordingly, it is a principal object of this invention to improve the
paper manufacturing process and quality of paper produced therefrom as
compared to the presently known processes and products.
One significant advantage of this invention is the reduction of brightness
reversion in lignin-containing pulps.
Another advantage of this invention is the low cost of manufacturing better
quality paper. The cost of manufacturing paper according to this invention
is only slightly higher than the cost of manufacturing untreated paper.
However, the invention provides a dramatic increase in the brightness
stabilization of lignin-containing paper.
A further advantage of this invention is that the compounds used to treat
the paper are known to be environmentally safe, and inexpensive.
Yet another advantage of this invention is that it reduces the quantity of
wood needed to make good quality paper because it provides the opportunity
for expanding the use of mechanical pulps which can be obtained from wood
in significantly higher yields than other pulps. Increasing the use of
mechanical and other lignin-containing pulps will produce more paper from
fewer logs. Also, the addition of brightness stabilization compounds adds
greater mass to the paper at a cost less than the cost of the same amount
by weight of pulp.
A still further advantage of this invention is that the presently known
paper manufacturing processes do not need to be significantly altered in
order to incorporate the invention. Known methods of production require
only a modification in the form of an additional step to treat the paper
sheet with the brightness stabilizing compounds.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 shows the effect of formate and CuSO.sub.4 or preventing yellowing
of lignin-containing pulps.
DETAILED DESCRIPTION
Pulps or paper are generally divided into two main classes being either
"chemical" pulps or "mechanical" pulps. The classification of the pulps is
determined by the manner in which the pulps are made from wood. Presently,
more chemical pulps are manufactured than mechanical pulps.
In producing chemical pulps, a chemical treatment is used to dissolve the
lignin from the wood. In these processes, most of the hemicelluloses are
dissolved. Thus, the pulp yield for a chemical pulping process is
typically 40%-50% of the wood. Mechanical pulping processes produce more
paper per unit of wood, with typical yields being higher than 85%.
Therefore, chemical pulps are characterized by lower yield and lower
lignin content.
Mechanical pulps, however, are characterized by their higher yield of pulp
and higher lignin content. Mechanical pulps require a significant amount
of mechanical energy such as grinding to break down wood chips made from
the wood. There are different subclasses of mechanical pulps. Groundwood
(GW) and pressurized groundwood (PGW) are generally manufactured by
pressing wood bolts against a revolving grinder wheel. Refiner mechanical
pulps (RMP) are made in disc refiners under ambient pressure and a
temperature of approximately 100.degree. C. Thermomechanical pulp (TMP)
are made in a process using high pressure steam to elevate the temperature
and, thus, soften the lignin making the fibers easier to separate.
Chemithermomechanical pulps (CTMP) and thermochemimechanical pulps (TCMP)
are made in a process which usually involves lignin sulfonation and high
temperature treatments to soften the chips. Chemimechanical pulps (CMP)
use a chemical treatment only and do not employ a thermal treatment.
The chemical class of pulps do contain some lignin, generally less than 5
weight percent as compared to more than 20% for mechanical pulps. The
lignin in chemical pulps is completely removed after a multi-stage
bleaching process is applied to achieve 90% brightness. However,
semi-bleached chemical pulps are not free of lignin. Therefore, certain
lignin-containing pulps fall into the chemical class as well.
In addition to the chemical and mechanical pulps, there is a quasi category
of pulps known as "semi-chemical". These pulps are hybrids of mechanical
and chemical pulps in that they have higher yields than the chemical pulps
(i.e., 50%-85% yield). However, the lignin content is higher than with
chemical pulps, and lower than the lignin content of mechanical pulps.
The major drawback to mechanical pulps, semi-chemical pulps, and chemical
pulps which are not fully bleached is that they contain lignin which, when
irradiated by light, can darken extensively. This limits the use of
lignin-containing pulps in various grades of quality printing papers.
During the manufacturing process of paper, the wood chips are broken apart
by one of the above methods and the fibers are dispersed in water to form
a slurry. The slurry is often bleached or whitened by known processes. The
type of bleaching or whitening process used in the manufacturing of
bleached lignin-containing pulp may be selected by the manufacture from
any of the standard known processes used to obtain the desired brightness
for said pulp. If the lignin-containing pulp is not bleached or mildly
bleached, it will usually be used as newsprint and will have a darker
quality. The pulp is bleached in a blend chest, bleaching tower or similar
vessel. It is then transferred to a papermaking machine. The slurry is
spread over a sheet-like or planar surface. Water is removed by
filtration, and the slurry is pressed into a sheet of the desired
thickness. The sheets are then dried. The sheets can be categorized by
their thickness type and are referred to as paper and/or pulp sheets.
The invention contemplates treating the pulp with a brightness-maintaining
compound or compounds just prior to or during the process step where the
pulp slurry is engaged in the papermaking machine, and subsequent to any
bleaching process, if such bleaching is employed.
The treatment of the paper sheet can take the form of dipping the sheet in
a solution of the brightness stabilizing compound(s) coating one or both
sides with the compound, or applying or spraying the compound in a
solution or solid form onto the sheet surface.
It is believed that the lignin-containing pulp is darkened by sunlight or
other irradiating light due to the presence of a photo-excited group
within the lignin. This photo-excited group, or free radical generated by
it, is believed to abstract a hydrogen atom from the lignin to form a
lignin radical. The lignin radical, in turn, reacts with oxygen. The
lignin radical plus oxygen forms colored materials. It is believed that
these colored materials contribute to the darkening of the paper.
Therefore, the invention comprises a method of manufacturing paper and
article of manufacture thereof with reduced brightness reversion qualities
whereby the lignin-containing paper or pulp sheet is treated with
compounds that are believed to donate a hydrogen atom to the photo-excited
group or free radical more easily than the lignin complex. The
photo-excited group or free radical is created by irradiating said sheet
with artificial or natural light. Compounds which are included in the
category of being good hydrogen donors are those compounds containing the
formyl functionality. Of the type:
##STR2##
wherein O is oxygen, C is carbon, H is hydrogen, and R is any functional
group having an element bonded to C, said element being selected from the
group consisting of nitrogen and oxygen. The compounds which satisfy this
class include sodium formate, calcium formate, magnesium formate, formic
acid esters such as hexyl formate, formamides such as N,N-dibutylformamide
N-formylurea, etc. Compounds with a very strong acidity (such as formic
acid) should be converted to a metal salt (such as sodium formate), or a
formyl ester, before application in the inventive process.
A 1985 survey conducted by the Canadian Center for Occupational Health and
Safety found no evidence to indicate that sodium or calcium formate is
toxic. These compounds are non-volatile with a melting point of more than
250.degree. C. They are also unreactive toward ground state oxygen and,
consequently, have been used for flame proofing of fibrous materials used
in gypsum board (Japan Patent No. 77-74,615--Kokai). Sodium and calcium
formates are considerably low in cost and are, at this writing, cheaper
than the bleached CTMP to which they might be added.
The amount of formyl compound to be used to coat or treat this sheet is
dependent upon the desired effect the manufacturer wishes to obtain. A
thicker coating generally will have a greater effect than a thin coating
of the same uniformity. However, even a very light treatment of formyl
compound will have effect on brightness reversion. It is up to the
manufacturer of the papermaking process to determine what percentage by
weight of the formyl compound should comprise of the paper sheet. Any
percent greater than 0% by weight would have some effect on brightness
reversion. The degree of brightness reversion reduction will be determined
by the extent of the coating.
Calcium carbonate will increase the hydrophobicity of the sheet surface.
This should enhance precipitation of the formate salt on the paper
surface. A higher concentration of formate salt on the paper surface
should enhance its ability to prevent brightness reversion. The carbonate
also might participate in beneficial free radical reactions as discussed
and shown below.
A formate salt such as sodium formate probably reduces light reversion by
the following reactions:
HCO.sub.2.sup.- .fwdarw.CO.sub.2..sup.- +H.
The CO.sub.2.sup.- radical is believed to react with O.sub.2 as shown
below.
CO.sub.2..sup.- +O.sub.2 .fwdarw.CO.sub.2 +O.sub.2..sup.-
K=2.4.times.10.sup.9 L/mole.s
In addition to the use of small amounts of calcium carbonate to increase
the effectiveness of formate salts, trace amounts of a superoxide anion
quencher (less than 0.5 weight percent) such as, copper sulfate or
ascorbic acid is used in conjunction with formate salt to treat the paper
sheet to reduce the darkening of the lignin-containing pulps. The results
of examples using the combination of formate salt and the superoxide anion
quencher showed a synergistic effect on the reduction of brightness
reversion. Ascorbic acid (0.1% on pulp) when used on lignin-containing
pulps by itself had absolutely no effect on reversion. The copper sulfate
when used on lignin-containing pulps alone actually increased the rate of
yellowing.
The superoxide anion (O.sub.2..sup.- is a free radical and probably
contributes to yellowing. When the carbonate is added to the pulp, there
then is a trace amount of HCO.sub.3.sup.- in the solution associated with
the paper or pulp. The bicarbonate anion might then convert the superoxide
anion to oxygen by the reactions below.
HCO.sub.3.sup.- .fwdarw.CO.sub.3..sup.- +H.
CO.sub.3..sup.- +O.sub.2..sup.- .fwdarw.CO.sub.3.sup.-2 +O.sub.2
K=4.times.10.sup.8 L/mole.s
It is believed that the Cu.sup.++ cation from the copper sulfate oxidizes
the superoxide anion to oxygen even better than the bicarbonate anion as
shown below. The copper cation is known as a superoxide anion quencher.
Cu.sup.+ + +O.sub.2..sup.- .fwdarw.Cu.sup.+ +O.sub.2
K=5.times.10.sup.9 L/mole.s
The copper cation may also react with some of the formate to form copper
formate which has a rate constant of 3.times.10.sup.8 L/mole.s for
reactions with the superoxide anion. Other superoxide anion quenchers
which could assist in trapping the superoxide anion include manganese (II)
compounds.
The following examples are given with the results obtained to illustrate
the activity of the hydrogen-donating compounds. Illustrations using
calcium carbonate, copper sulfate and ascorbic acid are also provided.
In all of the examples set forth below, the light-aging process was
accelerated by placing pulp sheets (relatively thick), or paper sheets
(relatively thin) close to a high intensity light source. The sheets were
placed in water-cooled compartments approximately 4.5 inches from a 1,000
watt mercury-tungsten lamp which emitted light at wavelengths of more than
300 nm. The experiments were conducted such that only half of the sheet
was reverted in the aging device. The other half of the sheet was reverted
in natural sunlight.
EXAMPLE 1
A peroxide-bleached TMP pulp sheet of 74.5% Elrepho brightness was used. It
was expected that this type of sheet would normally revert approximately
10 and 15 brightness points after one and three-hours, respectively, of
exposure in the experimental light-aging device. A solution of sodium
formate and water was prepared wherein the sodium formate was
approximately 50% of the weight of the pulp. The sheet was submerged in
the solution, removed, and allowed to dry. The sheet was then placed in
the light-aging device and the initial brightness and the brightness after
irradiation were measured. The formate-treated pulp reverted less than
what was expected for non-treated pulp as indicated by Table I.
TABLE I
______________________________________
Bleached TMP
Treated With 50%
Sodium Formate
Untreated
______________________________________
Initial Brightness, %
74.7 74.7
Brightness loss after 1 hour
3.8 10.0
Brightness loss after 3 hours
7.3 15.0
______________________________________
EXAMPLE 2
TMP was made from Norway spruce which was bleached to 74.7% Elrepho
brightness with hydrogen peroxide. Four 10-gram handsheets of pulp were
made in a 15-cm diameter Buchner funnel and pressed to 40% consistency
(i.e., 10-grams oven-dried pulp comprised of 25 grams of pulp plus water).
Varying amounts of sodium formate were dissolved in 20 mL of water and
added uniformly to the pulp sheet. The amounts of sodium formate used are
described in Table II.
TABLE II
______________________________________
Sample
1 2 3 4
______________________________________
Initial Brightness, %
74.7 75.3 75.4 75.4
Brightness Loss (1 hour)
12.1 11.7 9.5 6.8
Brightness Loss (3 hours)
16.0 15.6 13.4 9.9
Final (3 h) Brightness, %
58.7 59.7 62.0 65.5
______________________________________
Sample 1: Bleached TMP - untreated
Sample 2: Bleached TMP + 1.0%
Sample 3: Bleached TMP + 5.0%
Sample 4: Bleached TMP + 10.0% NaHCO.sub.2
It can be seen from Table II that the bleached, untreated lignin-containing
pulp had an initial brightness loss upon exposure to light. The first hour
exposure resulted in a 12.1-point brightness loss. The addition of 10%
sodium formate to the pulp resulted in a reversion of only 6.8 points
after one-hour. It is further seen that even 1.0% sodium formate applied
to the pulp gives a slight improvement in brightness stability as shown by
Sample 2.
EXAMPLE 3
Example 3 illustrates that the formate addition is effective in conjunction
with other standard known pulp modifications. The particular modification
employed was the standard known alkylation of phenolic hydroxyl groups in
the pulp. Accordingly, bleached TMP was alkylated with dimethyl sulfate
and sodium hydroxide. The phenolic hydroxyl concentration was measured by
aminolysis (Gellerstedt and Lindfors, Svensk Papperstid, 87 R115 [1984])
and by periodate oxidation in which methanol was formed. The results in
Table III show that although alkylation reduced reversion by approximately
33%, when alkylation was combined with a formate addition of 10%, the
results were a 58% decrease in reversion after one-hour of aging (Table
III).
Thus, the invention is effective in reducing the brightness reversion of
pulps which are manufactured in conjunction with other pulp modifications.
TABLE III
______________________________________
Sample
1 2 3
______________________________________
Initial Brightness, %
76.0 76.4 76.7
Brightness Loss (1 hour)
10.1 6.8 4.2
Brightness Loss (3 hours)
15.4 11.1 8.6
Final (3 h) Brightness, %
60.6 65.3 68.1
Phenolic Hydroxyl 0.16 0.02 --
Concentration, mole/g pulp
______________________________________
Sample 1: Bleached TMP
Sample 2: Bleached TMP + Alkylation
Sample 3: Bleached TMP + Alkylation + 10% NaHCO.sub.2
EXAMPLE 4
Bleached Norway spruce TMP was made from a different log than the pulps in
Examples 1-3. The purpose of this example is to illustrate the beneficial
effect of sodium formate with pulp when the formate concentrates on the
surface of the sheets. Sample 1 in Table IV corresponds to the bleached
TMP without any formate additive. Samples 2 and 3 correspond,
respectively, to 5% and 10% sodium formate uniformly added to the pulp
according to the process as described in Examples 1, 2, and 3. Samples 4
and 5 were prepared by pressing 10-gram handsheets to 40% consistency and
adding 10%-20% ground sodium formate crystals to the surface of the sheet.
The solid formate penetrated the sheet as it dried. As can be seen by
comparing Samples 3 and 4, the use of 10% sodium formate as a solid
resulted in only a slightly more effective reduction of reversion than did
the same amount of formate distributed uniformly throughout the sheet.
This indicates that the solid formate quickly dissolved on the sheet
surface and then diffused through the sheet before it dried. It can be
seen from the results illustrated in Table IV that 20% sodium formate
applied to the pulp as a solid (and, therefore, the surface) decreased
reversion from the untreated TMP value of 11.1 to 5.8 after one-hour of
irradiation.
TABLE IV
______________________________________
Sample
1 2 3 4 5
______________________________________
Initial Brightness, %
73.3 73.7 74.1 73.6 73.7
Brightness Loss (1 hour)
11.1 10.6 8.6 8.1 5.8
Brightness Loss (3 hours)
15.7 15.4 13.8 12.5 9.4
Final (3 h) Brightness, %
57.6 58.3 60.3 61.1 64.3
______________________________________
Sample 1: Bleached TMP
Sample 2: Bleached TMP + 5% NaHCO.sub.2 (solution)
Sample 3: Bleached TMP + 10% NaHCO.sub.2 (solution)
Sample 4: Bleached TMP + 10% NaHCO.sub.2 (solid)
Sample 5: Bleached TMP + 20% NaHCO.sub.2 (solid)
EXAMPLE 5
A sample of CTMP, a stronger form of pulp than TMP, was used. Samples 1-5
in Table V were prepared similarly to the samples described in Table IV
above.
Table V shows that when 10% sodium formate is added as a powder (Sample 4),
it is slightly more effective in reducing brightness reversion than when
it is added uniformly throughout (Sample 3). It can be seen that 20%
sodium formate placed on the pulp surface decreases the reversion from
12.4 to 4.9 after one-hour of irradiation. This represents an approximate
60% decrease in brightness reversion.
TABLE V
______________________________________
Sample
1 2 3 4 5
______________________________________
Initial Brightness, %
74.1 73.9 74.4 74.2 74.3
Brightness Loss (1 hour)
12.4 11.9 9.7 7.2 4.9
Brightness Loss (3 hours)
19.6 18.0 15.3 13.5 9.8
Final (3 h) Brightness, %
54.5 55.9 59.1 60.7 64.5
______________________________________
Sample 1: Bleached CTMP
Sample 2: Bleached CTMP + 5% NaHCO.sub.2 (solution)
Sample 3: Bleached CTMP + 10% NaHCO.sub.2 (solution)
Sample 4: Bleached CTMP + 10% NaHCO.sub.2 (solid)
Sample 5: Bleached CTMP + 20% NaHCO.sub.2 (solid)
EXAMPLE 6
Example 6 illustrates the effect of increasing the formate concentration on
the sheet surface using calcium formate. Calcium formate is less soluble
in water than sodium formate and, hence, should precipitate on the sheet
surface more easily than sodium formate. Calcium formate powder in the
amounts of 10% and 20% by weight based on pulp weight were sprinkled on
the surface of bleached CTMP, Samples 2 and 3, respectively. The CTMP was
the same as that used in Example 5 except that the CTMP of Example 5 had
been bleached with a lower concentration of hydrogen peroxide than the
pulps used in this Example. The calcium formate penetrated the sheet
initially but then crystallized on the sheet surface upon drying The
interaction between the calcium formate and the fibers did not
subjectively seem as good as sodium formate. Table VI shows, however, that
the addition of 20% calcium formate to the pulp surface decreased the
reversion from 14.1 to 10.2 points after one-hour of irradiation.
TABLE VI
______________________________________
Sample
1 2 3
______________________________________
Initial Brightness, %
76.2 75.7 76.3
Brightness Loss (1 hour)
14.1 11.5 10.2
Brightness Loss (3 hours)
19.1 15.0 13.8
Final (3 h) Brightness, %
57.1 60.7 62.5
______________________________________
Sample 1: Bleached CTMP
Sample 2: Bleached CTMP + 10% Calcium formate (solid)
Sample 3: Bleached CTMP + 20% Calcium formate (solid)
EXAMPLE 7
Example 7 seeks to concentrate the formate compound on the surface of the
pulp sheet. This was attempted by increasing the hydrophobic qualities of
the pulp sheet surface prior to the sodium formate addition. A 10-gram
pulp sheet was made and pressed in a 15-cm diameter Buchner funnel. 5%
calcium carbonate (0.05 gram) was slurried on top of the sheet with the
vacuum off. The sheet was de-watered under vacuum and pressed to 40%
consistency in a pneumatic press. Sodium formate was ground and sprinkled
on the surface of the sheets. Using this technique, it was not possible to
produce a carbonate coating that was completely uniform. Therefore, four
different runs using three different CTMP's were performed. In each run,
the bleached carbonate-coated and the carbonate plus sodium formate coated
sheets were reverted. The results are shown in Table VII and are typical
for all four runs.
TABLE VII
______________________________________
Sample
1 2 3
______________________________________
Initial Brightness, %
73.5 75.3 74.4
Brightness Loss (1 hour)
14.6 10.9 6.1
Brightness Loss (2 hours)
18.3 14.1 8.8
Brightness Loss (3 hours)
20.5 15.4 10.1
______________________________________
Sample 1: Bleached CTMP
Sample 2: Bleached CTMP + 5%
Sample 3: Bleached CTMP + 5% CaCO.sub.3 + 10% NaHCO.sub.2 (solid)
After one-hour of irradiation, the reversion was 14.6 points for the
bleached, untreated CTMP, 10.9 points for the pulp treated with carbonate
alone, and only 6.1 points for the sheet coated with the carbonate and the
formate. The three-hour reversion of the pulp treated with the carbonate
and formate was significantly less than the one-hour reversion for the
untreated, bleached pulp. After two-hours of artificial aging, the
bleached sample reverted 18.3 points, while the bleached CTMP sheet coated
with formate and carbonate reverted 8.8 points (see Table VII). This
represents a 52% decrease in reversion.
The bleached, untreated CTMP reverted 19.2 points in natural daylight after
eight-hours of irradiation, while the carbonate-coated sheet reverted only
16.6 points, and the carbonate plus formate coated sheet reverted only
11.1 points. (See Table VII(a)).
TABLE VII(a)
______________________________________
Sample
1 2 3
______________________________________
Initial Brightness, %
73.5 75.3 74.4
Brightness Loss (8 hours)
19.2 16.6 11.1
Final (8 h) Brightness, %
54.3 58.7 63.3
Samples: Same as Table 6
Irradiated by Natural Daylight
______________________________________
Temperature Conditions
______________________________________
8:30 A.M. 4.degree. C.
Dry Bulb 39.degree. F.
Wet Bulb 34.2.degree. F.
10:30 A.M.
6.degree. C.
Dry Bulb 42.degree. F.
Wet Bulb 36.5.degree. F.
1:00 P.M. 13.degree. C.
2:30 P.M. 14.degree. C.
4:30 P.M. 15.degree. C.
______________________________________
EXAMPLE 8
Examples 1-7 were conducted on thick sheets (weight approximately 500
g/m.sup.2) of pulp. However, in Example 8, thin sheets (60 g/m.sup.2) were
used. A standard-type draw down coater was used to coat the thin sheets
made from a commercial bleached CTMP. To concentrate the coating on the
paper surface, a concentrated solution of the calcium carbonate and sodium
or calcium formate was required. Using a solution that consisted of 30% by
weight calcium carbonate, 30% by weight sodium formate, and 40% by weight
H.sub.2 O, a coated sheet of 83.9% brightness was produced (bleached pulp
brightness, 75.8%). This pulp reverted only 2.0 points after three-hours
in the light-aging device. However, the coating solution was of a viscous
nature and, therefore, the coating charge was approximately two times the
weight of the paper (both sides were coated).
To decrease the amount of hydrogen-donating compound coating the paper
surface, a less concentrated solution was used. However, when less
concentrated solutions were used, the additives quickly penetrated the
sheet and "filled" the sheet rather than coating it, as desired. The
limited brightness increase (typically less than 1 point) which was
afforded by coating the surface in this manner indicates that penetration
of the additive occurred. These filled or penetrated sheets showed a
significant improvement in brightness stability as compared to untreated
sheets. It appeared that when calcium carbonate was dissolved in formate
solution, calcium formate was as effective as sodium formate. However,
calcium formate offers an advantage over the sodium salt with regard to
price stability in the marketplace. Both salts are made by reacting the
corresponding hydroxide with carbon monoxide. Unlike sodium hydroxide,
calcium hydroxide is generally inexpensive and readily available.
The supply of calcareous materials from which calcium oxide can be easily
made appears unlimited. The results from the "filled" or penetrated sheets
are shown in Table VIII.
TABLE VIII
______________________________________
Sample
1 2 3
______________________________________
Initial Brightness, %
75.8 76.4 76.2
Brightness Loss (1 hour)
7.8 5.6 2.9
Brightness Loss (3 hours)
13.9 10.4 5.5
Final (3 h) Brightness, %
61.9 66.0 70.7
______________________________________
Sample 1: Bleached CTMP
Sample 2: Bleached CTMP + 31%
Sample 3: Bleached CTMP + 43% CaCO.sub.3 + 6% Ca(HCO.sub.2).sub.2
EXAMPLE 9
Example 9 illustrates the synergistic effect of the addition of a
superoxide anion quencher, such as copper sulfate.
Trace amounts of copper sulfate were added with the sodium formate to the
pulp surface (50-100 ppm on pulp). Cu.sup.++ oxidizes the superoxide anion
which contributes to yellowing as follows:
Cu.sup.++ +O.sub.2.sup.- .fwdarw.Cu++O.sub.2
K=5.times.10.sup.9 L/mole.s
Solutions were made from sodium formate and from sodium formate and copper
sulfate. These solutions were added uniformly to thin sheets made from
bleached CTMP. The increase in light absorption coefficient measured at
457 nm versus time of light-aging is shown in FIG. 1. The untreated sheet
showed a rapid initial yellowing followed by a slower phase that followed
zero order kinetics as reported by Lebo (Ph.D. Thesis, Institute of Paper
Chemistry, 1988). The formate (20% by weight) decreased this rapid initial
yellowing and copper sulfate (80 ppm Cu on pulp) enhanced the effect still
further. However, copper sulfate by itself actually increased the rate of
yellowing. The increase in the light absorption coefficient after one-hour
of light reversion was 2.6, 1.0, and 0.7 m.sup.2 /kg, respectively, for
the untreated formate-treated, and formate plus copper sulfate-treated
sheets.
Therefore, to further illustrate the synergism between sodium and calcium
formate and other chemicals capable of quenching the superoxide anion, new
sheets were made from another bleached CTMP. These sheets were treated
with formate solution containing trace amounts of either copper sulfate or
ascorbic acid, both of which have a high rate constant
(1.25.times.10.sup.6 L/moles.s) for their reactions with superoxide anion.
All of the solutions in this experiment were adjusted to pH 6. The results
in Table IX clearly show the synergistic effects of formate and either
copper sulfate or ascorbic acid. Ascorbic acid by itself (0.1% on pulp)
had no effect on reversion.
TABLE IX
______________________________________
Sample
1 2 3 4 5
______________________________________
Initial Light 1.27 1.19 1.22 1.38 1.30
Absorption Coeff. (K)
Increase in K 2.81 1.61 0.86 1.18 1.48
(2 h Light-Aging)
______________________________________
Sample 1: Bleached CTMP
Sample 2: Bleached CTMP + 20% NaHCO.sub.2 (solution)
Sample 3: Bleached CTMP + 20% NaHCO.sub.2 + 80 ppm
Sample 4: Bleached CTMP + 20% NaHCO.sub.2 + 0.1% Ascorbic Acid
Sample 5: Bleached CTMP + 20% Ca(HCO.sub.2).sub.2 + 80 ppm Cu.sup.+
EXAMPLE 10
The same pulp sheets as described in Example 9 were treated with 20 weight
percent of three other formyl compounds, hexyl formate,
N,N-dibutylformamide and N-formylurea The compounds, with the exception of
N-formylurea, are liquids with high boiling points. However, they are
volatile at room temperature. Hexyl formate in duplicate experiments
evaporated completely in 2.5 hours. All three compounds were dissolved in
methanol and added uniformly to the sheets. Most of the methanol
evaporated from the sheets in about 30 minutes. Methanol treatment of the
sheets without formyl compounds had no effect on reversion. Table X shows
the results of light-aging for Samples 2, 3 and 4, compared to the
untreated Sample 1.
TABLE X
______________________________________
Sample
1 2 3 4
______________________________________
Initial Light 1.27 1.24 1.20 1.27
Absorption Coeff. (K)
Increase in K 2.81 2.25 1.74 2.54
(2 h Light-Aging)
______________________________________
Sample 1: Bleached CTMP
Sample 2: Bleached CTMP + 20% hexyl formate
Sample 3: Bleached CTMP + 20% N,Ndibutylformamide
Sample 4: Bleached CTMP + 20% Nformylurea
The foregoing examples can be repeated with similar success. From the
foregoing description, one skilled in the art can easily ascertain the
essential characteristics of this invention and, without departing from
the spirit and scope thereof, can make various changes and modifications
to the invention in order to adapt it to various usages and conditions.
Without further elaboration, it is believed one skilled in the art can,
using the preceding description, utilize the present invention to its
fullest extent. The above-mentioned embodiments are, therefore, to be
construed as merely illustrative and not limitative of the remainder of
the disclosure whatsoever.
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